There may be several reason why your muscles may always be tight. It could be as simple as your diet, stress, medications, inadequate rest or even wear and tear from exercise. If you know that you are mindful of these areas it may be time to have a look at your muscular system at whole. As a muscle activation technique specialist, I see all sorts of muscle imbalances that cause early muscle fatigue, pain and limitations. When a muscle tightness up it is the bodies natural response to protect the joint. The question to ask is why your body is locking up to protect the joint?
During my training to become a M.A.T specialists one phrase was drilled in to our minds as we look at the muscular system and that was “muscle tightness is secondary to Muscle weakness”. What this is saying is that the reason a muscle tightens to protect the joint is because there is another muscle across that joints axis that is weak. For example, if your hamstrings are tight and is limiting motion in flexion (Picking the leg strait up) one or more of the muscles that move that joint into its position is weak. Therefore, the hamstrings tighten up because one or more of the hip flexors are weak.
If you suspect you are having muscle weakness across the joint axis do some light isometrics opposite of the muscle that is tight. An example for this would be if the hamstrings are tight, do isometrics for your hip flexors to get the muscles to contract and move the leg into the range vs stretching the hamstring to get more flexibility. If you would like to know specific muscles and how to train those muscles, a muscle activation technique specialist in your area can find out which muscles are weak and improve their contraction capabilities. This process will not only aid in a flexibility program but will also improve range of motion as well as stability and strength.